In Italy, 6 January has long been a date associated with unusual goings-on. Marking the Feast of the Epiphany in the Christian calendar, it is on this day each year that families across the peninsula awake to find that they have been visited by the Befana – a witch-like old lady who flies into town on a broomstick and slips down people's chimneys before – in the manner of Santa Claus – leaving coal for the naughty kids and treats for the nice ones.

But if the existence of airborne OAPs and well-behaved children might seem implausible to some, then it was the events that unfolded in Serie A on Sunday that truly stretched the bounds of credibility. This was a day on which struggling Pescara defeated high-flying Fiorentina and Genoa recorded their first home win since August, and yet neither of those turned out to be the most surprising result of the afternoon.

Even before kick-off you would have struggled to find anyone who gave Sampdoria a chance of beating Juventus in Turin. The Blucerchiati had finished 2012 level with Pescara in 15th place, just two points clear of the relegation zone. Their opponents were eight points clear at the top of the table. In nine home games, Juve had scored 19 and conceded just five.

If the upset always seemed improbable, then by the 31st minute it had become impossible. Not only were Sampdoria a goal behind, Sebastian Giovinco converting a spot-kick mid-way through the first half, but now they were also down to 10 men. Gaetano Berardi, the same player who had given away the penalty, was shown his second yellow card after clattering into Andrea Barzagli.

The Sampdoria forward glanced nervously towards the touchline. Delio Rossi had only been appointed as manager a few days before the winter break, and this was just his second match in charge, so nobody really knew how he would react to losing his right wing-back. Even with the team trailing, he would not have been the first to pull off a forward to shore up the defence.

In the event, though, Rossi chose not to make any immediate substitutions. Instead he pulled his remaining wing-back, Marcelo Estigarribia, back into the defence, shifting Daniele Gastaldello out to the right and converting what had begun the game as a 3-5-2 into something approaching a 4-3-2. At half-time he then replaced Estigarribia with Lorenzo De Silvestri, switching responsibilities among his remaining defenders but retaining the same basic shape.

Even if it had made sense tactically to withdraw Mauro Icardi at that point, Rossi might still have thought twice about it. The forward was, after all, only present in Turin because his manager had demanded it – reversing the club's previous commitment to let Icardi represent Argentina's Under-20 side in a World Cup qualifying tournament in South America.

Icardi, anxious to nail down his place for the finals this summer, had been promised by Sampdoria's former sporting director, Pasquale Sensibile, that he would be allowed to take part. It was not until a few days before Christmas that Rossi convinced him otherwise. With all three of Maxi López, Nicola Pozzi and Eder missing through injury, Icardi was just about the only fit striker that Rossi had left.

Eder had since recovered to face Juventus, but he was more a creator than a goalscorer. Rossi had seen enough of Icardi already to know that this kid represented the best chance his team had of taking what few chances Juventus might allow them.

Yet another product of the Barcelona academy, Icardi had arrived in Sampdoria two years earlier with a prolific record at youth level. Born in Lionel Messi's hometown, Rosario, he had moved to the Canary Islands with his family at nine years old before capturing the attention of a number of leading European sides with his prolific goalscoring for the local club, Vecindario. So anxious were Barcelona to land him that they sent Messi himself to put in a good word.

Icardi did not disappoint, scoring 38 goals in two years with the Catalan club's youth team, but increasingly it became clear that this was not the place for him to make his breakthrough. Not only was the competition for places fierce, but Icardi did not fit the Barcelona mould. A classic centre-forward who models his game after that of long-time idol Gabriel Batistuta, Icardi was more interested in finishing moves than helping to build them.

So it was that he made the switch to Samp in January 2011, on a loan deal that would soon be made permanent. After another productive season at youth level, Icardi was called up to the senior team – then playing in Serie B – in May 2012 and marked his debut with a winning goal against Juve Stabia.

This season he had become an increasingly regular fixture in the starting XI, though his only goal prior to this weekend had arrived in the derby win over Genoa. The adjustment to top-flight football had proved challenging for a player who is still not yet 20, though he was also not helped by being constantly deployed as a lone striker in a team who have struggled to get the ball up the field.

Rossi hoped that the presence of Eder up front would help to get Icardi running in behind defenders, rather than always having to play with his back to goal. Seven minutes into the second-half in Turin on Sunday, Rossi got his wish – albeit not in the manner he had envisaged.

It was nothing more clever than a long ball forward from Nenad Krsticic – gifted possession cheaply by Andrea Pirlo – which released Icardi down the right. Catching up to the ball on the edge of the area with Federico Peluso in hot pursuit, the striker opted for a first-time shot across goal. Lacking in any great venom, Icardi's effort should have been saved easily enough but instead hopped up off the turf in front of the diving Gigi Buffon and bounced slowly into the net.

"I messed up because when the shot was taken I was still backpedalling, so my balance was a bit precarious," Buffon later explained. "Usually I don't make these sorts of mistakes, but this time I was too far forwards."

Even so, the goalkeeper was not unduly concerned – acknowledging later that he still expected his team-mates to find a winner. Instead, 13 minutes later he was picking the ball out of his net again. This time it was Pedro Obiang who released Icardi with a simple through-ball, and the striker crashed his near-post effort into the roof of Buffon's net.

A stunned silence engulfed Juventus Stadium, one that would be replaced soon enough by throaty angst as Mirko Vucinic first hit the bar, then jabbed wide from a yard out. But instead of relief there would be only more misery for the home support, as Claudio Marchisio left the game on a stretcher in injury time. They would be grateful, at least, to hear the club confirm later on that the player had suffered only a deep bruise.

The game finished 2-1. It was Juve's first league defeat with Antonio Conte present on the touchline, the two previous losses to Milan and Internazionale having both occurred during his four-month suspension. "We are not martians," he told reporters afterwards. "Everything we have done up to now has been achieved through ferocity and determination. This defeat must help us to grow."

Sampdoria will hope that victory can help them to do the same. The change in formation was not Rossi's only innovation, the manager also having brought Angelo Palombo back into the fold after six months in which he had been completely marginalised. More often deployed in midfield in the past, Rossi fielded Palombo in the middle of his three-man defence.

But the greatest hope after this game will be that it will mark the moment at which Icardi begins to realise his potential at this level. "I would never have imagined that I would score two goals past Buffon – a player who I admired as a kid," said the striker. "On the first one he gave me a hand, but the second one had to go in where I put it."

Icardi may or may not belive in the Befana. But he certainly seems to have no lack of faith in himself.

Results and talking points

• The biggest story of the winter break in Italy was Kevin-Prince Boateng's remarkable walk-off after being subjected to racist abuse during Milan's friendly against Pro Patria. Silvio Berlusconi has since insisted that his team would repeat the act even in the most high-profile of games, regardless of Sepp Blatter's feelings on the matter, and public reaction to the gesture has been overwhelmingly positive.

And yet just two days on from that initial incident, further racist chanting was heard at Rome's Stadio Olimpico on Saturday night during Lazio's win over Cagliari – with Victor Ibarbo the main target. Other sections of the stadium were heard to react against the abuse, which did eventually die down, but the impact had already been felt. "Racist chants against Ibarbo at the Olimpico …" tweeted his team-mate Mauricio Pinilla afterwards. "You never learn anything …!!! #Shame"

• As for Pro Patria themselves, a prominent "No racism" banner was hung from the railings before their game at Savona. There had been reports that fans might paint their faces black in an intended show of solidarity, but in the end they thought better of it. It would not have been without precedent, though. Treviso's players famously took such a step in support of their team-mate Akeem Omolade after he was abused back in 2001.

• Changing tack somewhat, the other big piece of news has been Giuseppe Rossi's arrival in Fiorentina from Villarreal. Rossi, who has not played a competitive game since 26 October 2011, represents an enormous gamble but certainly one which could pay huge dividends. Prior to injury he was valued a lot higher than the €10-12m Fiorentina are reported to have paid. He is not expected back before March at the earliest, though Fiorentina could have done with him on Sunday as they failed to find a way past an inspired Mattia Perin in the Pescara goal.

• With Inter also losing 3-0 away to Udinese, this week's big winners at the top of the table were Lazio, whose 2-1 win in that game over Cagliari moved them up to within five points of Juve and four points clear of the Viola, and Napoli, who crushed Roma 4-1 at the Stadio San Paolo to move back up to third. Not for the first time, this was all about Edinson Cavani, who scored a blistering hat-trick and has now scored a ridiculous 25 goals in 23 games across all competitions this season. Afterwards the Napoli owner Aurelio De Laurentiis was up to his usual mischief, saying it was his "dream" to one day pair Cavani with Mario Balotelli up front.

• "Wherever he goes, Mauro [Zarate] will be loved and appreciated by the Lazio fans," said the player's agent, Luis Ruzzi, last Wednesday. And so on Saturday those supporters proved him wrong, unveiling a series of banners which read: "The true champion is humble. He goes and collects the balls when training with the reserves. He doesn't cry on Twitter, and he reduces his wages. He does not cling on to an overly generous contract. Zarate: leave."